1. Field of the Invention
The present invention relates to an exhaust gas purifying filter that is made of a ceramic material and is used for collecting particulate matter, such as fine carbon particles, emitted from internal combustion engines.
2. Description of the Related Art
Use of a honeycomb structure made of ceramic material is viewed as promising as an exhaust gas purifying filter that collects, and then burns, the particulate matter, such as fine carbon particles, that are emitted by a diesel engine or the like, so as to be regenerated.
Specifically, as shown in FIG. 3, an exhaust gas purifying filter 9 of the prior art has cells 90 of a honeycomb structure of which is alternately stopped with a plug 95. On each end of the honeycomb structure, the plugs 95 are arranged in a checkerboard pattern. Walls 98 are provided with a catalyst supported thereon.
With this constitution, as shown in FIG. 3, exhaust gas 8 that has entered the cells 90 is discharged after passing through the walls 98, while the particulate matter included in the exhaust gas 8 is captured in the walls 98 so as to accumulate therein. The particulate matter that has accumulated is burned and removed by the action of the catalyst supported on the walls 98, so that the exhaust gas purifying filter 9 is occasionally rejuvenated.
In order to further improve the purification performance of the exhaust gas purifying filter 9, it is effective to increase the inner surface area of the filter. This has been achieved by various methods such as decreasing the thickness of the walls of the honeycomb structure, that constitutes the exhaust gas purifying filter, and decreasing the pitch of the cells, among others. However, although the methods based on decreasing the thickness of the walls are effective, there is a limitation as the methods make it difficult to manufacture, and decrease the strength of, the structure.
As other means to increase the filtering area than decreasing the thickness of the walls, it is considered to be effective to use the plugs 95 as part of the filter.
In consideration of the problems of the prior art described above, the present invention has an object to provide an exhaust gas purifying filter that can utilize the plugs as part of the filter.
A first aspect of the invention is an exhaust gas purifying filter made as a honeycomb structure comprising a multitude of cells each surrounded by walls, each of the cells being stopped with a plug on one end thereof.
The walls and the plugs of the exhaust gas purifying filter are all made of a porous material, and the value of M=(L/t)xc3x97(P1/P2) is in a range of 10 less than M less than 90, where P1 is the porosity of the walls, P2 is the porosity of the plugs, t is mean thickness of the walls and L is mean length of the plugs.
In the exhaust gas purifying filter of the present invention, the walls and the plugs are all made of the porous material, with the porosity and the size thereof satisfying the particular relationship.
That is, the value of M calculated as (L/t)xc3x97(P1/P2) is in a range of 10 less than M less than 90, where P1 is the porosity of the walls, P2 is the porosity of the plugs, t is mean thickness of the walls and L is mean length of the plugs. Accordingly, the exhaust gas purifying filter can be made in such a way that the exhaust gas can pass through the plugs while maintaining sufficient strength of the plugs. As a result, the plugs can be used as part of the filter without decreasing the strength of the plugs, so that the entire filtering area can be increased and the performance of the exhaust gas purifying filter as a whole can be improved.
Mean values are used for t and L according to the present invention. The value of porosity can be given by measuring the volume of the pores, by a mercury injection method, using a porosimeter. Since the mean values are used for t and L, it is not necessary for all the plugs to satisfy the requirements described above. It suffices that at least the mean values provide for the particular range of the value of M.
A second aspect of the invention is an exhaust gas purifying filter made in honeycomb structure comprising a multitude of cells each surrounded by walls, each of the cells being stopped with a plug on either end thereof.
The walls and the plugs of the exhaust gas purifying filter are all made of a porous material, wherein at least 50% of the plugs have values of M2=(L2/t)xc3x97(P1/P2) in a range of 10 less than M2 less than 90, where P1 is the porosity of the walls, P2 is the porosity of the plugs, t is the mean thickness of the walls and L2 is the length of each plug.
In the exhaust gas purifying filter of the present invention, at least 50% of the plugs have values of M2, calculated from the values of P2 and L2 of each of the plugs, the porosity P1 of the walls and the mean thickness t of the walls, that fall within the particular range described above. This constitution can also improve the purifying performance by utilizing the plugs as part of the filter.